My grades tipped over last year. I was a slacker. Then I got back on my feet from slacker land and now I get really good grades like I did last year (fourth grade).
Never leave homework till the last minute, especially if you have a bed time and you're doing something special that day. Do it when you get the chance to. Even when you're absolutely sure that you'll get another chance later in the day, do it. It saves stress for when it's the last minute and if you do a little extra work, your teacher will be proud of you. Like Chatterbox said, make a list of priorities, relax your mind if you're stressed, talk to an adult you can trust...everything they said. The "deal with your parents" one is a very good idea. Maybe if you get good grades, then you get...extra added to your allowance? A new little toy? A piece of candy? Eating at your favourite restaurant? Your favourite food? Anyhoo, rewards make you determined to get good grades. If your teacher gives you some time to work on homework, then use it wisely. Don't talk. If you laugh with your friends during instructional times a lot, don't even make eye contact. What if they make funny faces that make you laugh? Don't pass notes, either. That distracts you from learning. If you're really, really behind, you could use times like quiet reading (if you have it) and instead of reading, work on homework. Or maybe recess. Or on the bus in the mornings and afternoons.
I wish you good grades.